1. Field of the Invention
The present invention concerns an automatic gain control in a receiver for a digital radio telephone in which the received signal is amplified with an RF amplifier, converted to a first intermediate frequency in a first mixer, potentially converted to a second intermediate frequency in a second mixer, and is thereafter conducted to two branches in which the base band frequency I and Q signals are produced.
2. Discussion of Related Art
In digital radiotelephones, such as in GSM radio telephones, automatic gain control is used to maintain the level of the received signal at a constant value prior to AD converters.
The most common way is to accomplish the AGC function in a way similar to that in AM or SSB receivers wherein the AGC is implemented using a voltage controlled amplifier controlled with analog voltage as the IF amplifier, said amplifier attenuating the signal more the higher its amplitude. In said manner known in the art, part of the AGC function can be placed in the first stages of the receiver.
An example of applying the AM or SSB receiver principles in the GSM radio telephone receiver is presented in FIG. 1. A radio frequency signal from the antenna ANT is filtered and amplified with an amplifier, this being possibly part of the AGC function, and mixed in a mixer 3' with the frequency of a first local oscillator 1.Lo. The produced mixing results are amplified and the signal filtered therefrom is taken to a second mixer 4' in which it is mixed with a second local oscillator frequency. The obtained mixing result is filtered using a filter 5' and is passed through two consecutive amplifier stages, the gain thereof being controlled with a voltage AGC from the D/A converter of the logic section, with the AGC function being thus implemented. After the AGC amplifiers, the signal path is divided into two branches in which the I signals and the Q signals are produced by mixing, in the mixers 6' and 7', the signal frequencies with a third local oscillator frequency, which is directed to the mixers with a 90 degree phase shift relative to one another.
According to the block diagram described above, a receiver can be designed, but a number of deficiencies are associated therewith. The properties of a voltage controlled AGC amplifier located in the intermediate frequency stages are in general poor due to the current controlled amplifiers used and the large control range required in the GSM system. The control curve of the AGC amplifier is non-linear and its shape, as well as the amplification of the amplifier, is dependent on temperature. Owing to said properties, great differences exist between different amplifier exemplars, as a result of which the control curve of the amplifier and the dependence of the properties thereof on temperature must, for each telephone, be tabulated individually or compensated for by means of complex analog circuits.
The amplification of the AGC amplifier should at maximum gain be great in order not to impair the signal/noise ratio when lowering the amplification. Because of the large amplification, it is, on the other hand, necessary to employ good filtering prior to the AGC amplifier because, otherwise, the signals in the adjacent channels will be amplified too much and will cause blockage and an intermodulation phenomenon. A filter presenting good enough characteristics is in general expensive and bulky.
Since in a known circuit the frequency of a local oscillator for use in producing I and Q signals is equal to the intermediate frequency, it is in general exceedingly difficult to prevent the local oscillator signal from becoming connected to the intermediate frequency amplifier. Such connection in itself is not objectionable but in the above circuit in which the amplification of the intermediate amplifier in a second intermediate frequency is changed, major problems arise regarding the d.c. voltage levels in the I branches and Q branches. The d.c. voltage levels in both branches are dependent on the gain of the AGC amplifiers at any one moment, and any change in the gain causes a rapid change in d.c.levels, the effect of which cannot be eliminated such as by means of an a.c. circuit (capacitors) in the I branch and the Q branch.